In nature, arsenic appears in many different formations. Very commonly, arsenic associates with iron and copper, but also with nickel, cobalt, gold and silver. Arsenic is also the most important impurity to be removed during recovery of non-iron metals. During pyrometallurgical processes the majority of the arsenic remains in fly ash of the waste heat boiler and the electric furnace. The utilization of arsenic has not increased in relation to its recovery, so the majority of arsenic has to be stored in the form of waste. Since arsenic and its compounds are toxic, they must be made into the least soluble form possible before being removed from the process. The most poorly soluble arsenic compounds at neutral pHs are for example zinc, copper and lead arsenates, but the binding of arsenic to these valuable metals has not been seriously considered precisely due to the valuable metal content of what will remain as waste. One arsenic precipitation method used a lot nowadays is to precipitate arsenic with iron as ferric arsenate, which is quite poorly soluble. In particular, the crystalline form of ferric arsenate, scorodite, FeAsO4.2H2O, is more poorly soluble than its other form, amorphous ferric arsenate.
The hydrothermal precipitation of arsenic as poorly soluble scorodite has been known for a long time. Scorodite formed hydrothermally at a temperature of over 150° C. is very poorly soluble and its arsenic content is in the region of 30%, so it is a fairly effective way to bind arsenic as a product that does not pollute the environment. Up until now, the greatest obstacle to use of the method has been the economic cost of the autoclave, since the method demands quite a large autoclave, because the arsenic concentration of the solution to be treated is generally rather low. The method is considered economically viable, if it has been possible to combine it with the oxidation of some valuable metal, such as gold concentrate. It is mentioned in U.S. Pat. No. 7,314,604 that no autoclave is known to be in use solely for the formation of scorodite.
Nowadays the possibility of preparing scorodite at temperatures of maximum 100° C. or at ambient pressure has been well researched. In the article by Wang, Q. et al: “Arsenic Fixation in Metallurgical Plant Effluents in the Form of Crystalline Scorodite via a Non-Autoclave Oxidation-Precipitation Process”, Society for Mining Metallurgy and Exploration, Inc, 2000, a method for removing arsenic from fly ash is described, where arsenic is recovered as scorodite. The first processing stage of the arsenic-containing material is the oxidation of trivalent arsenic (As(III)) to pentavalent (As(V)) with a gas containing sulphur dioxide and oxygen in acidic conditions, in which arsenic does not precipitate. After this, arsenic precipitation is performed in atmospheric conditions, in which the Fe(III)/As(V) molar ratio is specified to be 1. Precipitation is carried out either in one or several stages, but precipitation as scorodite demands the over-saturation of the solution, which is achieved by recycling scorodite crystals to the first reactors of the precipitation and simultaneously neutralising the suspension. The preferred pH zone is in the region of 1-2 and this is maintained by feeding a suitable neutralising agent into the precipitation stage. In these conditions arsenic can be precipitated to the level of 0.5 g/l. The final arsenic removal to the level of below 0.1 mg/l is done by means of a second purification step, in which the iron and arsenic Fe(III)/As(V) molar ratio is adjusted to the area of 3-5 and the pH to 3.5-5. The amorphous precipitate generated in this stage is routed back to the first precipitation stage, where it dissolves and precipitates again as scorodite.
It is stated in the article by Singhania, S. et al: Acidity, Valency and Third-Ion Effects on the Precipitation of Scorodite from Mixed Sulfate Solutions under Atmospheric-Pressure Conditions, in Metallurgical and Materials Transactions B, Vol. 37B, April 2006, pp. 189-197, that the precipitation occurring in atmospheric pressure conditions should be performed as a controlled crystallisation, which results in well-crystallised scorodite. Crystallisation depends on the pH of the precipitation solution and its control as well as the amount of iron and arsenic and their ratio in the solution.
A method is described in U.S. Pat. No. 6,406,676 for removing arsenic and iron from an acidic solution generated in the hydrometallurgical processing of concentrate. Arsenic and iron precipitation are performed in two stages, whereby the pH is kept in the range of 2.2-2.8 in the first precipitation stage and in the range of 3.0-4.5 in the second stage. Lime is routed to both precipitation stages and, in addition, air is added to the second. A separate iron-arsenic precipitate exits each stage, and the precipitate from the second stage is recycled to the first stage, whereby any remaining unreacted lime can be utilised in the first stage. The precipitate from the second stage can also be recycled to the beginning of the same stage in order to improve the crystallisation of the precipitate. According to the example, the method has been applied for a zinc-containing solution and it is noted there that zinc does not precipitate with the iron and arsenic, but can be recovered after this treatment. The temperature at which the precipitations were carried out is not mentioned in the patent, but they presumably take place at ambient pressure. Nor does the publication mention the form in which the iron and arsenic are precipitated.
US patent application 2008233023 refers to Japanese application 2005-161123, in which a method for removing arsenic from soot is described. According to the method, arsenic-containing soot is dissolved in an acidic solution and precipitated as amorphous iron arsenate by mixing an iron-containing acidic solution into the first solution. The amorphous iron arsenate generated is crystallised by heating the mixed solutions. The crystallised iron arsenate is removed by filtration. An elevated temperature of 95° C. is mentioned in the Japanese application, so the process is performed at atmospheric pressure.
The prior art described above provides a good picture of current arsenic precipitation methods and the trend seems to be for the removal of arsenic from solution or precipitate by turning it into scorodite at atmospheric pressure. The drawback of the methods is that the formation of very poorly soluble scorodite, in which the arsenic concentration is high, is still uncertain at atmospheric pressure, or it demands highly controlled conditions. The hydrothermal formation of scorodite by routing iron- and arsenic-containing solutions directly into an autoclave has proved a financially costly solution, even though scorodite is the most poorly soluble arsenic compound possible. The solubility of scorodite in the US Environmental Protection Agency's TCLP test (Toxicity Characteristic Leaching Procedure) is below 5 mg/l.